This invention concerns a method for the controlled pre-rolling of thin slabs leaving a continuous casting plant.
To be more exact, the method according to the invention is carried out immediately downstream of the foot rolls of a mould on thin slabs leaving the mould performing continuous casting of thin slabs.
By thin slabs are meant slabs with a width from 800 to 2500 mm. or more and with a thickness from 25 to 90 mm.
The invention is applied advantageously, but not only, to slabs having a final thickness between 30 and 60 mm. at the outlet of the continuous casting machine.
The invention can also be applied to the continuous casting of billets, whether they be round, square, rectangular, etc.
The invention can be applied to straight and curved continuous casting plants.
EP-A-625.388 in the name of the present applicants discloses a controlled pre-rolling method whereby a thin slab is subjected to a pre-rolling action in a zone downstream of the foot rolls.
This document provides for the performance of a controlled pre-rolling with reduction with a liquid core, or soft reduction, of the slab leaving the crystalliser so as to produce a slab with a reduced thickness at the end of the casting machine.
As is disclosed in that prior art document, the main advantages of the controlled pre-rolling are the ability to obtain at the outlet of the casting machine a slab with a slender thickness (30-60 mm.) when using a crystalliser of a greater thickness, and also the ability to obtain a refining of the structure of solidification of the metal and the elimination of the central segregation in the slab.
The soft reduction, if it is to be effective, has to take place with a continuous controlled reduction of the thickness of the slab, and this condition can be achieved with a substantially tapered conformation of the segment of slab subjected to the soft reduction.
The above prior document states that this tapered segment has to have a length between about 0.8/1 meters and about 7 meters, in which the greater length corresponds to the end of the containing zone produced by idler containing rolls included downstream of the crystalliser and of the foot rolls.
Moreover, the pre-rolling method described in that document is based on an ON LINE solidification model which, on the basis of the actual casting conditions, determines the exact profile of solidification of the slab.
This method arranges to reduce in a controlled and required manner the thickness of the slab with a liquid core according to the main casting parameters (speed of extraction, difference of temperature in the tundish, secondary cooling downstream of the mould, type of steel, thicknesses) which are partly set at the start-up of the process and are partly continuously monitored by suitable sensors and/or transducers.
The rolls which perform the pre-rolling are associated with load cells and/or pressure transducers, which monitor the pressure exerted on the thin slab and check that this pressure, for each pair of rolls or for each assembly of pairs of rolls, corresponds to the pressure which the dynamic programme sets for obtaining the desired effect on the slab and for performing the desired pre-rolling.
Moreover, each pair of rolls is associated with a position transducer which monitors the distance or gap between the opposed rolls of each pair.
In this case too the task of the position transducer is primarily to check that the positioning of the pairs of rolls or of the assemblies of pairs of rolls corresponds to the value which the program sets for that pre-rolling cycle.
The whole system is governed by a data processing unit controlling the pre-rolling which receives signals from pressure transducers and position transducers, which may be transducers working with assemblies of rolls or with single rolls, and also from monitors of the speed of the slab, monitors of the secondary cooling parameters and monitors of the temperature of the molten metal in the tundish and of the temperature of the slab leaving the crystalliser.
There may also be included a monitor to identify the presence or absence of a liquid core within the slab.
The data processing and control unit of the prior art document processes these parameters and provides the pairs of rolls in real time with the best adjustment values.
By rolls are meant here continuous rolls, rolls covering sectors, belts, etc. that is to say, any system of the state of the art.
With the pre-rolling assembly are associated means for the secondary cooling of the slab which consist, for instance, of a plurality of sprayer nozzles.
Moreover, at least one descaling assembly may be associated with the pre-rolling assembly.
U.S. Pat. No. 5,018,569 discloses a pre-rolling method whereby the slab leaving a mould and the foot rolls is caused to cooperate with a plurality of pairs of successive rolls so as to achieve the desired reduction of thickness with a liquid core.
This document of the prior art arranges to identify and to determine the end of the liquid cone by reading the pressure exerted by the rolls on the slab passing through.
To be more exact, the adjustment of the hydraulic pressure of the actuator jacks associated with the rolls acting on the slab is set by a control element so as to obtain the desired reduction of thickness of the slab passing through.
However, in that document the opposed rolls of a pair are separated by stationary spacers and therefore can approach each other until the supports abut against these spacers.
Therefore, when the position of these spacers has been determined, the machine can no longer be adjusted and therefore cannot be adapted to the specific requirements which may occur from time to time.
Moreover, the adjustment and clamping in position of the rolls based only on the adjustment of the pressure of the actuator jacks is very unstable and not at all reliable.
In fact, the required pressure which the rolls have to exert on the slab changes during the casting steps in particular because in the various steps the temperature and the speed cannot always remain perfectly constant; these changes of pressure cannot be foreseen in so exact a manner as to be able to be balanced with variations of pressure in the adjustment actuator jacks and thus to enable the relative roll to remain in the desired position corresponding to that pre-rolling pass.
If the rolls do not remain fixed in the desired position, cracks and bends may be caused in the slab being processed.
In the case of this document of the prior art, when the pressure of the actuator jacks exceeds a given value, the speed of extraction is altered so as to alter the length of the liquid cone within the slab.
The alteration of the extraction speed, however, entails a plurality of drawbacks.
First of all, an alteration of the speed has an effect on all the working parameters and makes necessary a re-programming and a re-adaptation of the whole casting machine according to that alteration.
Furthermore, an alteration of speed may entail consequences regarding the quality of the product and the output of the plant.
Besides, each change of speed entails transient conditions which may alter the planar condition of the meniscus and therefore the surface quality of the cast product.
Moreover, the alteration of speed may be slow in terms of adaptation times, and this fact may entail a delay in reaching the conditions of the best disposition.
A further shortcoming of systems known to the state of the art, as has been demonstrated, is that the regulation of the position of the pre-rolling elements present downstream from the foot rolls, which is based exclusively or mostly on values obtained ON LINE and used in a dynamic programme continuously up-dated with the results of the monitoring, cannot always give satisfactory results.
In particular, in some cases there have been delays in adapting and regulating times, difficulties in obtaining the required precision, and malfunctionings deriving from the large amounts of calculations which have to be done every time one of the parameters being monitored is changed.
This means that extremely complex software has to be used, as the software has to elaborate a very large amount of data, re-calculate in real time the correct values of the reduction in thickness which has to be applied according to the parameters monitored, and then supply the output data to regulate and adapt a very high number of devices, all of which makes the machine too reactive and instable.